Lt Col Alexander Vindman, director for European affairs at the national security council, who was on the president’s July 25 phone call with the leader of Ukraine, said it was “improper” for him to demand a foreign government investigate a US citizen and political opponent.
It was for that reason he raised his concerns about the president’s request to Volodymyr Zelensky.
“I never thought I would be sitting here testifying in front of this committee and the American public, about my actions. When I reported my concerns, my only thought was to act properly and to carry out duty,” Lt Col said on Tuesday.
He added: “I want to take a moment to recognise the courage of my colleagues who have appeared and are scheduled to appear before this committee.”
The officer also appeared to directly rebut an an attack Mr Trump made last Friday on social media against Marie Yovanovitch, a former US ambassador to Ukraine. He said everywhere she had served “went bad”.
“I want to state that the vile character attacks on these distinguished and honourable public servants is reprehensible,” said Lt Col Vindman.
“It is natural to disagree and engage in spirited debate, this has been our custom since the time of our Founding Fathers, but we are better than callow and cowardly attacks.”
The testimony of the 44-year old officer, who told the committee he spoke Russian, Ukraine and a “little bit of English”, carries particular weight because was was among a handful of officials who listened directly as Mr Trump spoke to Mr Zelensky.
Republicans have denounced previous witnesses as having no direct knowledge and relying on “hearsay”.
“The uniform I wear today is that of the United States army. The members of our all volunteer force are made up of a patchwork of people from all ethnicities, religions, and socio-economic backgrounds who come together under a common oath to protect and defend the constitution of the United States of America,” said Lt Col Vindman.
“We do not serve any particular political party, we serve the nation. I am humbled to come before you today as one of many who serve in the most distinguished and able military in the world.”
Democrats launched a formal impeachment inquiry after a whistleblower alleged Mr Trump had improperly asked Mr Zelensky to launch an investigation into Joe Biden and his son, in exchange for the release of US military aide and a state visit to Washington.
Mr Trump has denied any wrongdoing and dismissed the proceedings as a “witch hunt”.